I primed the undersides of the models black, while I could still reach those areas, before attaching the figures to bases. There's my gravel mix, for adding texture to the base.
I glued zinc, fender washers to the bottom of the base, to add heft, lower the center-of-gravity, and enable non-foam transport, via magnetic trays. I experimented with Liquitex modeling paste, in place of the spackling compound.
The modeling paste cracked (lower left). The model does not adhere to the modeling paste any better than the spackle. Both compounds are about the same softness/hardness when cured.
All else equal, with the modeling paste prone to cracking, I did the rest of the models using the spackling compound.
I experimented with copper for the hardware, but I reverted to bronze or brass. Copper makes more sense from an engineering standpoint, I think. Lighter than bronze, as far as I know, and cheaper. But a yellow color is going to work better for my color scheme than an orange-ish color. I could opt for a brown color, which could probably represent any aged copper-based metal, but I think the hardware would fade too much in the background then. I'm going to try to split the difference and shade down this yellow base-coat with olive and brown.
Airbrush is done. On to brushwork.